| Phase shifter is a kind of microwave control circuit which can changes an input signal’s phase to a new one. Phase shifters are the most essential components in the microwave systems that have traditionally been limited to military applications or high-end commercial applications due to cost constraints. Digital phase shifters are more immune to control voltage noise and temperature variation than analog phase shifters, so the digital phase shifter used in active phased array antennas (APAA) becomes increasingly extensively. Microwave phase shifters have mostly been implemented with III-V planar integrated circuit (IC) technologies. However, the significant progress made in the wide-band-gap semiconductor technology now offers the potential for a superior high power, compact, low cost phase shifter. More specially, the third generation of semiconductor devices which represented by emerging Gallium Nitride (GaN) HEMTs has the excellent characteristics of operating in extreme temperatures (～600℃), exhibiting high breakdown voltages (～100 V) and high current densities (～1.5 A/mm) at the same time, and is the most important choice of fabricating high performance digital phase shifter MMICs. So it has great practical significance to conduct research on phase shifter mechanism and design GaN based phase shifter MMIC.Under this research background, results on the digital phase shifter is presented in this thesis, including the analysis of the basic phase shifting mechanism and various topologies of phase shifter circuit and the design of the phase shifter circuits. On the basis of theory analysis of the GaN HEMT switch devices, we offer the suggestions for the selection of switch devices applied to phase shifters. Then a single-pole double-throw switch circuit for high-pass/low-pass filter phase shifter is designed. On the basis of theory analysis of phase shifter, an X-band 5 bits digital phase shifter is designed by using the measurement data of self-developed GaN HEMT. The main results of the thesis are as follows:1. We perform small signal measurements of self-developed GaN HEMT switch devices. The switch performance of GaN HEMTs with different gate width is characterized both in series and parallel configuration. Then the suggestions for the selection of switch devices applied to phase shifters are introduced. We use ADS to design a single-pole double-throw switch circuit for high-pass/low-pass filter phase shifter using the S-parameters of the GaN HEMT switch device:According to the procedure of the GaN MMIC process, the SPDT circuit layout is properly designed using software L-Edit.2. The topology circuit of each bit is determined according to the consideration of the switch performance of GaN HEMT devices and the design specification.11.25°/22.5° phase bits adopt the load-line configuration.45° phase bit employ the all-pass network while the 90° and 180° phase bits both exploit the high-pass/low-pass network. We use ADS to perform design and optimization and employ extensive EM simulation for each bit circuit until the design requirements are met. After that, the 5-bit phase shifter cells are cascaded and simulated. On the condition of overall performance, we choose one cascading order of 5-bit circuits whose chip size is smallest. According to process rules of the GaN MMIC product line,5-bit layout circuit is properly designed. Finally, the complete layout of the 5-bit phase shifter is design in L-Edit and the layout size is 5000μm×2900μm. The simulation results of 5-bit phase shifter achieve the desired objective:Over the 9-10 GHz operating frequency range, the designed 5-bit phase shifter’s phase error is -2.917～3.194° and the worst root-mean-square (RMS) phase error is 1.1.565°; the insertion loss is 14.372-17.126dB while the RMS amplitude error is smaller than 0.541dB; the input and output return loss are smaller than -13.5dB and-14.1dB, respectively. The simulation results show that the performance of the designed 5-bit phase shifter has met the design specification. |
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